1. Field of the Invention
This invention relates to an ion-selective electrode which is useful for potentiometrically determining the ionic activity level (or concentration) of a specific ion contained in an aqueous liquid, especially in the body fluids (blood, urine, saliva or the like) of an organism.
2. Description of the Prior Art
A method for determining the ionic activity level of a specific ion contained in drop of a liquid (tap water, river water, sewage, industrial drainage or the like) or in body fluids (blood, urine, saliva or the like) by using a sheet-like ion-selective electrode is known. This method is performed by spotting a test liquid and a reference liquid onto the surfaces of ion-selective layers of a pair of ion-selective electrodes. These ion-selective electrodes are electrically separated from each other. The liquids thus spotted come into contact with each other through a bridge disposed between these ion-selective electrodes so as to make electric conduction. The potential difference across the ion-selective electrodes is measured. The ionic activity level of the test liquid is determined from this measurement. A device for determining ionic activity levels by this method is disclosed in U.S. Pat. Nos. 4,053,381 and 4,273,639 and the like.
In general, an inner reference electrode which comprises a conductive layer and a slightly soluble salt Japanese Unexamined Patent Publication Nos. 56(1981)-33537 and 57(1982)-186163 is preferable. The silver chloride layer may be formed on the silver layer by electrolytic oxidation of the silver layer or vapor deposition of silver chloride. A silver bromide layer and a silver iodide layer can be formed in a manner similar to the method for forming silver chloride layer.
As the quaternary ammonium salt, which is a well-known ingredient of the halide ion-selective layer, a tetraalkylammonium salt having a long-chain alkyl group, for example, trioctylmethylammonium chloride (e.g. "Aliquat 336" produced by Aldrich Chemical Company) can be used.
In order to form the halide ion-selective layer, the quaternary ammonium salt and the polymer which is used as the binder may be dissolved in a solvent, which can dissolve them relatively well, to form a solution of appropriate concentration, and then this solution may be applied to or sprayed upon the silver halide layer (e.g. silver chloride layer) and dried.
As the binder, polyvinyl chloride or a vinyl chloride copolymer such as the one disclosed in U.S. Pat. No. 4,555,274 may be used. Also, various vinyl acetals, e.g. vinyl formal, vinyl acetal (acetal with acetaldehyde) or vinyl butyral are acceptable. In particular, vinyl butyral is preferable. As the organic polymer, a homopolymer of vinyl acetals or a copolymer of a vinyl acetal with another monomer which can be copolymerized with vinyl acetal may be used. The layer, and an ion-selective layer are essential to the sheet-like ion-selective electrode. A water-soluble salt layer is often disposed between the slightly soluble salt layer and the ion-selective layer. Usually, these layers are mounted on a non-conductive, water-impermeable support so that the ion-selective layer (or an additional layer) is the outermost layer.
A chloride ion-selective layer, in which a silver layer, a silver chloride layer and a chloride ion-selective layer containing a hydrophobic polymer as a binder are mounted in this order is well-known and is disclosed, for example, in U.S. Pat. No. 3,591,482.
In general, a halide ion-selective layer comprises a quaternary ammonium salt (e.g. "Aliquat 336" produced by Aldrich Chemical Company) and a hydrophobic organic polymer which is used as a binder. As the hydrophobic organic polymer used as the binder, ployvinyl chloride, a vinyl copolymer as disclosed in U.S. Pat. No. 4,555,274, the polyvinylacetal species disclosed in Japanese Unexamined Patent Publication No. 62(1987)-169044 or the like, are known.
However, in the ion-selective electrodes using such halide ion-selective layers, the voltage across a pair of electrodes cannot accurately reflect the ionic activity level of a halide ion unless the ion-selective layer contains a means, e.g. an attached mask, for limiting the surface area over which the liquid comes into contact with the halide ion-selective layer.